Method of improving the color tone of a recording layer containing photoconductive lead (ii) oxide

ABSTRACT

The color tone of a recording layer containing photoconductive lead (II) oxide is improved by treatment with an alkali metal halide, alkaline earth metal halide or onium halide salt.

United States Patent Tavernier et al.

[ 5] Feb. 15, 1972 METHOD OF IMPROVING THE COLOR TONE OF A RECORDING LAYER CONTAINING PHOTOCONDUCTIVE LEAD (II) OXIDE Inventors: Bernard Ilippoliet Tavernier, Edegem; Al-

ions Jozef De Meyer, Schelle; Johannes Josephus Vanheertum, l-Iallezandhoven,

all of Belgium Assignee: Gevaert-Agia N.V., Mortsel, Belgium Filed: Nov. 12, 1969 'Appl. No.2 876,062

Foreign Application Priority Data Nov. 12, 1968 Great Britain ..53,67 2/68 US. Cl. ..96/l.5, 250/65 ZE, 252/501 0 Int. Cl ..G03g 5/00, G03g 7/00 Field of Search ..96/l .5; 252/501; 250/65 ZE Primary Examiner-George P. Lesmes Assistant Examiner-John R. Miller Attorney-William J. Daniel ABSTRACT The color tone of a recording layer containing photoconductive lead (ll) oxide is improved by treatment with an alkali metal halide, alkaline earth metal halide or onium halide salt.

5 Claims, No Drawings METHOD OF IMPROVING THE COLOR TONE OF A RECORDING LAYER CONTAINING PHOTOCONDUCTIVE LEAD (II) OXIDE This invention relates to improved photoconductive substances, improved recording materials containing said substances, and a process for preparing them.

It is known to use photoconductive materials for the recording of electromagnetic radiation patterns in terms of differences in charge density or electroconductivity.

Electrophotographic materials consisting of a support, on which a photoconductive substance is coated together with a binder are generally known in this respect.'As photoconductive substances in such coatings specific inorganic as well as organic substances can be used, e.g., sulfur, selenium, and compounds belonging to the type of the oxides, sulfides, and selenides of zinc, cadmium, mercury, antimony, bismuth, titanium, and lead, anthracene and a great variety of other organic monomeric and polymeric photoconductors.

Among these photoconductors photoconductive zinc oxide has been selected for its brilliant white color whereby it constitutes an excellent contrasting base for developer materials applied thereon.

The reason why other photoconductive metal oxides, which have a photosensitivity as interesting as zinc oxide, have not.

been used in copying materials but only in recording plates suited for development with powder and transfer thereof to a receiving material, is to be sought in their color tone that is not acceptable for contrasting with thedeveloping material. With regard thereto reference is made to photoconductive lead(II) oxide of the tetragonal type having a color ranging from red to tan and which is particularly suited for X-ray recording (ref. US. Pat. No. 3,008,825). The lead (II) oxide of the orthorhombic type havinga yellow color is utilized for the same purpose according to the US. Pat. No. 3,266,932.

The present invention provides a method for improving the contrast in color tone to black substances of photoconductive recording materials on the basis of photoconductive lead(II) oxide.

According to the present invention the lead(II) oxide is treated before or after its incorporation into a photoconduc tive recording material with a compound, preferably a salt, which reacts with the lead(II) oxide and forms on its surface a product having a color, which in the chromaticity diagram is closer to the chromaticity point of daylight than the untreated lead(II) monoxide.

The description of the said chromaticity point in a chromaticity diagram can be found in the book: The Science of Color, published by the Optical Society of America, Washington 36, DC. fifth ed. (1966) p. 244-246.

It has been found that the contrast of lead(II) oxide, particularly red or tan lead(II) oxide in respect of black substances can be improved by a treatment with a halide being better water soluble than lead chloride.

Preferred halides, which give a more neutral color tone to lead( 11) oxide, in particular to the red or tan type, are alkali metal halides, alkaline earth metal halides, or onium halides, particularly ammonium halides.

Preferred salts in that respect are ammonium chloride, ammonium fluoride, ammonium iodide, sodium chloride, sodium bromide, potassium chloride, calcium chloride, magnesium chloride, barium chloride, strontium chloride, calcium bromide, magnesium fluoride, barium bromide and strontium bromide. Most interesting is calcium chloride, which covers the red to tan lead(ll) oxide with a bright white product that masks the inherent color of the photoconductor.

Without restricting the invention thereto, it is assumed that with calcium chloride the following reaction takes place:

The halide is preferably applied in an aqueous liquid in highly concentrated dissolved form, e.g., in a concentration of at least percent by weight. The interaction with the colored photoconductor can be accelerated by heating the treating solution and/or photoconductor.

The treatment can be applied before as well as after the exposure and does not markedly reduce the chargeability and the X-ray sensitivity of the lead(II) oxide. The sensitivity to visible light, however, is reduced but not to such an extentthat the recording material becomes useless for recording electromagnetic radiation of this range of the wavelength spectrum.

In order to have profit of the inherent sensitivity in the visible range of the electromagnetic spectrum of the lead(ll) oxide, particularly the red or tan 1ead( II) oxide, the decolorizing treatment is performed in a step subsequent to the development of the electrostatic image. This treatment can be effected by simply dipping the developed photoconductive recording material in an aqueous solution containing a said halide or a mixture thereof. The contacting of the decolorizing liquid with the recording material can, however, be carried out according to any coating technique, e.g., spraying, coating, whirling, by using a lick-roller, etc.

In said treatment the interaction with the lead(II) oxide is more or less effective or takes more or less time depending on the type of binding agent. Indeed, the binding agent may not prevent wetting of the lead(II) oxide by the liquid improving the contrast of the lead(II) oxide in respect of the developing substance(s). Satisfactory results have been obtained by means of binding agents that are not very hydrophobic. 0ptimal results have been obtained hitherto with polyvinyl acetate as binding agent. Wetting can be improved by means of common wetting agents and/or by applying organic solvents that are miscible with-water, preferably those penetrating into the binder.

Wetting can also be improved by applying a voltage to the recording layer. In this respect reference can be made to the U.K. Pat. specification Nos. 1,020,505 and 1,033,419. It has to be noted that the binder material when perfectly suitable from the viewpoint of not preventing the wetting of lead(II) oxide, should also be electrically insulating to the extent that an electrostatic charge applied to the recording layer must not be conducted by the binder at a rate that prevents the formation and retention of an electrostatic charge thereon for photographic processing.

The recording materials of use in the present invention can be prepared by any of the known processes for preparing binder-pigment coatings. Thus, the pigment-binder composition together with a suitable solvent for the binder may be flowed on a base material or otherwise coated on the base by dipping, whirling, spraying, using a doctor blade, a dip roller, etc.

For preparing a very homogeneous pigment dispersion the coating composition should preferably contain a dispersing agent preventing coagulation or flocculation of the lead(II) oxide during storage and coating of the dispersion. In organic solvents acid esters of oxyacids of phosphorus, e.g., monobutyl phosphate, have proved to be particularly suited for dispersing lead(II) oxide.

The photoconductive pigment-binder composition for use according to the present invention may contain all kinds of substances. applied in zinc oxide electrophotography and prefcrably contains 50 to percent by weight of lead(II) oxide calculated on the binder.

It is self-explanatory that the lead(II) oxide of the present invention can be mixed with other known photoconductive substances preferably those being white or only slightly colored, e.g., in order to obtain an increase in sensitivity for a particular part of the electromagnetic spectrum, e.g., photoconductive zinc oxide and other photoconductive chalcogenides and the ingredients of photoconductive coatings described in the UK. Pat. specification Nos. 1,020,504 and 1,020,506 can be used.

In view of the object of the present invention the photoconductive substances which are treated to obtain a more neutral color tone can be combined with fluorescing agents and/or brightening agents as described e.g., in Phot.l(orr. 94, No. l (l958)p. 3-11 and No. 2 (1958) p. 19-26.

The photoconductive recording materials on the basis of lead( ll) oxide treated according to the invention are particularly suitable for visible light continuous tone reproduction and for X-ray recording with or without intensifying screens such as lead screens. The electrostatic charge is preferably applied by means of a positive corona charging, since optimal results can be obtained therewith. A double corona charging may be applied, whereby there is understood that both sides of the recording material are corona-charged with charges of opposite sign.

The physical shape or conformation of the electrostatic recording material may be widely different. It may be a plate, a sheet, or a drum, and it may be flexible as well as rigid.

In the manufacture of photoconductive recording materials treated according to the present invention and used in a recording technique based on the production of an electrostatic charge irnage, a relatively conductive support is used for the recording layer. For instance an electroconductive sheet or plate, or an insulating sheet or plate covered with an electroconductive interlayer can be employed. By electroconductive plate or sheet there is meant a plate or sheet, the electrical resistivity of which is smaller than that of the photoconductive layer i.e., preferably at least 100 times smaller.

Suitable conductive plates or sheets can be made of metals such as aluminum, zinc, copper, tin, iron, or lead.

Suitable electroconductive interlayers for insulating supports are, e.g., vacuum-coated metal layers such as silver or aluminum layers, conductive polymer layers e.g., applied from polymers containing quaternized nitrogen atoms such as those described in the U.K. Pat. specification No. 950,960, or layers containing in a binder dispersed particles e.g., carbon black and metal particles. The binder used for said particles preferably has a resistivity lower than ohm-cm.

Paper sheets, which have an insufficient electrical conductivity are coated or impregnated with substances enhancing their conductivity e.g., by means of a conductive overcoat such as a metal sheet (e.g. aluminum) laminated thereto.

Substances suitable for enhancing the conductivity of a paper sheet and which can be applied in the paper mass are hygroscopic compounds and antistatic agents as described, e.g., in the U.K. Pat. specification No. 964,877, and antistatic agents of polyionic type, e.g., a quaternized polyethylene imine.

When the photoconductive composition is coated from organic solvents on paper sheets, the latter are preferably made organophobic i.e., are impermeabilized for organic solvents, e.g., by means ofa water-soluble colloid covering layer or by a strong hydration of the cellulose fibers as in glassine paper.

Electrostatic images in recording materials used according to the present invention can be developed according to one of the techniques known in electrophotography, wherein use is made of the electrostatic attraction or repulsion of finely divided colored substances.

If a continuous tone reproduction has to be made, elec trophoretic development is preferred. In this case use is made of finely divided substances dispersed in an insulating liquid, e.g., a hydrocarbon liquid.

The recording materials applied in the present invention can also be developed electrolytically.

The following examples illustrate the present invention.

EXAMPLE 1 A photoconductive recording layer containing 80 percent by weight of tetragonal lead(ll) oxide, dispersed in polyvinyl acetate applied to an aluminum plate was treated for 15 sec. at C. in a 25 percent by weight aqueous solution of calcium chloride. The tan color of the recording layer became masked with a white product strongly adhering thereto.

The thus treated recording layer was charged with a positive corona and exposed to the bone skeleton of a hand by means of a medical X-ray tube of 52 kv./l00 ma.sec. placed at a distance of 50 cm.

The residual charge image was detected by electrophoretic development.

The developer was prepared by diluting the concentrated developer composition described hereinafter in a volume ratio of 15/],000 by means of SHELLSOL T (trade name for a hydrocarbon solvent):

carbon black (average particle size: 20 nm.) 30 g.

lecithine L5 g. SHELLSOL T (trade name) 750 cc. resin solution prepared as described hereinafter g.

The resin binder solution was prepared by heating at 60 C. 500 g. of ALKYDAL L67 (trade name of Farbenfabriken Bayer A.G., Leverkusen, W.-Germany, for a linseed oil modified (67 percent by weight) alkyd resin and 500 cc. of white spirit containing 11 percent by weight of aromatic compounds till a clear solution was obtained. Subsequently the solution was cooled.

A black continuous tone image of the hand was obtained on a white background.

EXAMPLE 2 Example 1 was repeated with the difference, however, that the treatment of the recording layer by means of the calcium chloride solution was carried out after the electrophoretic development.

The result was the same as that obtained in Example 1.

We claim:

1. A method of improving the color tone of a recording layer containing photoconductive lead(ll) oxide particles dispersed in a resinous binder which comprises treating the surface of said layer in the presence of water with an alkali metal halide, an alkaline earth metal halide or an onium halide salt which is more water soluble than lead chloride; the amount of said halide being sufficient to combine with said lead(ll) oxide at said layer surface and modify the color thereof nearer to the chromaticity point of daylight in a chromaticity diagram.

2. A method according to claim 1, wherein the halide is ammonium chloride, ammonium fluoride, ammonium iodide, sodium chloride, sodium bromide, potassium chloride, calcium chloride, magnesium chloride, barium chloride, strontium chloride, calcium bromide, magnesium fluoride, barium bromide or strontium bromide.

3. A method according to claim 1, wherein the treatment is carried out by means of an aqueous solution containing at least about 25 percent by weight of said salt.

4. A method according to claim 1, wherein the treatment takes place before informationwise exposure to active electromagnetic radiation of said recording layer.

5. A method according to claim 1, wherein the treatment takes place after the charging, informationwise exposure and development of a charge image on the photoconductive recording layer. 

2. A method according to claim 1, wherein the halide is ammonium chloride, ammonium fluoride, ammonium iodide, sodium chloride, sodium bromide, potassium chloride, calcium chloride, magnesium chloride, barium chloride, strontium chloride, calcium bromide, magnesium fluoride, barium bromide or strontium bromide.
 3. A method according to claim 1, wherein the treatment is carried out by means of an aqueous solution containing at least about 25 percent by weight of said salt.
 4. A method according to claim 1, wherein the treatment takes place before informationwise exposure to active electromagnetic radiation of said recording layer.
 5. A method according to claim 1, wherein the treatment takes place after the charging, informationwise exposure and development of a charge image on the photoconductive recording layer. 